Recombinaison Dynamics photovoltaïques en couches minces de matériaux par résolution temporelle Micro-ondes Conductivity
Summary
A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.
Abstract
A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.
Introduction
Flash-photolysis time-resolved microwave conductivity (FP-TRMC) monitors dynamics of photo-excited charge carriers on the ns-µs timescale, making it an ideal tool for investigating charge carrier recombination processes. Understanding the decay mechanisms of photo-induced charge carriers in thin film semiconductors is of key importance in a range of applications, including photovoltaic device optimization. The induced carrier lifetimes are often functions of induced carrier density, excitation wavelength, mobility, trap density and trapping rate. This paper demonstrates the versatility of the Time Resolved Microwave Conductivity (TRMC) technique for investigating a wide range of carrier dynamic dependencies (intensity, wavelength, microwave frequency) and their interpretations.
Photogenerated charges can modify to both the real and the imaginary parts of the dielectric constant of a material, depending on their mobility and degree of confinement/localization1. The conductivity of a material 
is proportional to its complex dielectric constant
where 
is the frequency of a microwave electric field, 
and 
are the real and imaginary parts of the dielectric constant. Thus, the real part of the conductivity is related to the imaginary part of the dielectric constant, and can be mapped onto microwave absorption, while the imaginary part of the conductivity (subsequently referred to as polarization) is related to a shift in the resonance frequency of the microwave field1.
TRMC offers several advantages over other techniques. For instance, DC photoconductivity measurements suffer from a range of complications arising from contacting the material with electrodes. Enhanced recombination at the electrode/material interface, back injection of charges through this interface, as well as enhanced dissociation of excitons and geminate pairs due to the applied voltage2 all lead to distortions in the measured carrier mobilities and lifetimes. In contrast, TRMC is an electrodeless technique which measures the intrinsic mobility of the carriers without distortions due to charge transfer across contacts.
A significant advantage of using microwave power as a probe for carrier dynamics is that, as well as monitoring the decay lifetimes of charge carriers, decay mechanisms/pathways can also be investigated.
TRMC can be used to determine the total mobility3 and lifetime4 of induced charge carriers. These parameters can subsequently be used to distinguish between direct and trap-mediated recombination mechanisms3,5. The dependence of these two separate decay pathways can be quantitatively analyzed as a function of carrier density3,5 and excitation energy/wavelength5. The localization/confinement of induced carriers can be investigated by comparing the decay of the conductivity vs polarizability5 (imaginary vs real part of dielectric constant).
Additionally, and perhaps most importantly, TRMC can be used to characterize trap states which act as charge carrier decay pathways. Surface traps, for example, can be distinguished from bulk traps by comparing passivated vs unpassivated samples6. Sub-bandgap states can be directly investigated using sub-bandgap excitation energies5. Trap densities can be deduced by fitting TRMC data7.
Due to the versatility of this technique, TRMC has been applied to study a wide range of materials including: traditional thin film semiconductors such as silicon6,8 and TiO29,10, nanoparticles11, nanotubes1, organic semiconductors12, material blends13,14, and hybrid photovoltaic materials3,4,5.
In order to obtain quantitative information using TRMC, it is crucial to be able to accurately determine the number of absorbed photons for a given optical excitation. Since methods for quantifying absorption of thin films, nanoparticles, solutions and opaque samples differ, the sample preparation and calibration techniques presented here are designed specifically for thin film samples. However, the TRMC measurement protocol presented is very general.
Protocol
Representative Results
Discussion
While the TRMC technique can offer a wealth of information about photoinduced charge carrier dynamics, this is an indirect measurement of conductivity, and therefore care needs to be taken when interpreting results. The TRMC technique measures total mobility, and cannot be used to distinguish between electron and hole mobilities. The underlying assumption that conductivity is proportional to change in reflected power holds only when that change is small (< 5%)16. Furthermore, if the shift in r…
Disclosures
The authors have nothing to disclose.
Acknowledgements
Acknowledgment is made to the Australian Research Council (LE130100146, DP160103008). JAG is supported via an Australian Postgraduate Award, and DRM by an ARC Future Fellowship (FT130100214). We thank Nikos Kopidakis for helpful discussions.
Materials
| Hellmanex III detergent | Sigma Aldrich www.sigmaaldrich.com/catalog/product/sial/z805939?lang=en®ion=AU |
Z805939 | Corrosive and toxic. See SDS. |
| Lead (II) iodide (99%) | Sigma Aldrich www.sigmaaldrich.com/catalog/product/aldrich/211168?lang=en®ion=AU |
211168 | Toxic. See SDS |
| Anhydrous dimethylformamide (99.8%) | Sigma Aldrich www.sigmaaldrich.com/catalog/product/sial/227056?lang=en®ion=AU |
227056 | Toxic. See SDS |
| Anhydrous dimethylsulfoxide (99.9%) | Sigma Aldrich www.sigmaaldrich.com/catalog/product/sial/276855?lang=en®ion=AU |
276855 | Toxic. See SDS |
| Anhydrous 2-Propanol (99.5%) | Sigma Aldrich www.sigmaaldrich.com/catalog/product/sial/278475?lang=en®ion=AU&gclid= COnlgPaw780CFQZvvAod17EA4Q |
278475 | |
| Methylammonium iodide | Dyesol www.dyesol.com/products/dsc-materials/perovskite-precursors/methylammonium-iodide.html |
MS101000 | Also sold by Sigma Aldrich |
| Poly(methyl methacrylate) | Sigma Aldrich | 445746 | |
| Anhydrous chlorobenzene (99.8%) | Sigma Aldrich www.sigmaaldrich.com/catalog/product/aldrich/445746?lang=en®ion=AU |
284513 | Toxic. See SDS |
| Equipment | Company | Model | Comments/Description |
| UV-VIS-NIR spectrophotometer | Perkin-Elmer | Lambda 900 | |
| Profilometer | Veeco | Dektak 150 | |
| Vector Network Analyzer | Keysight www.keysight.com/en/pdx-x201927-pn-N9918A/fieldfox-handheld-microwave-analyzer-265-ghz?cc=US&lc=eng |
Fieldfox N9918A | |
| Tunable wavelength laser | Opotek www.opotek.com/product/opolette-355 |
Opolette 355 | |
| Neutral density filters | Thorlabs www.thorlabs.hk/newgrouppage9.cfm?objectgroup_id=3193 |
NUK01 | |
| Power meter | Thorlabs www.thorlabs.com/thorproduct.cfm?partnumber=PM100D |
PM100D | |
| Power sensor | Thorlabs www.thorlabs.com/thorproduct.cfm?partnumber=S401C |
S401C | |
| Cavity | Custom built | The cavity used in for this experiment was designed and built in-house. |
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